Because we are rebellious children. We do it in so many ways every single day that it's just second nature to us now...or first nature, really.
Think about how many people think dogs, cats and birds belong indoors when clearly they were designed and thrive in an outdoor setting....they have fur and feathers, their natural diet can be found out there, the soils, rain, plant life and the very air contributes towards their ongoing good health and life. They were designed to withstand extreme temps and those that could not died and left those that could to breed more that could.
Same with humans...we weren't meant to stay indoors draped over a couch all day or work inside for most of our lives, eat processed and low nutrient foods, lead sedentary lifestyles, stare at a TV all the time or any of those unhealthy living practices. We were designed to live in contact with the soils, to be outdoors quite a bit but still take shelter indoors in extreme weather as we are lacking in fur, scale and feathers, and we were designed to be active. Very active.
Nope. We know better and so we do the contrary and force our own ideas on a perfect design.